This invention relates to microwave field detectors.
The measurement of electromagnetic energy is required for determining leakage of electromagnetic energy, testing of equipment for meeting performance standards, surveying for hazardous fields and other applications such as would be required for experimental purposes. One of the most important needs is in the area of leakage monitoring for possibly hazardous fields in the vicinity of high power microwave sources such as microwave ovens and radar.
Present methods of monitoring make use of antenna-detector configurations and associated metering equipment. This method is often inaccurate because the probe can disturb the field and the need for metering increases the complexity and cost for this approach.
Another approach uses a spherical film bolometer to absorb the incident energy and change the pressure within the sphere. The change in pressure produces a movement of an indicating fluid in a capillary attached to the sphere. The movement is a measure of the field intensity. Some of the drawbacks to these methods are the cost of construction, the instrumentation required and difficulties with operation such as the sticking of the indicating fluid and disturbance of the field by the instrument.
The use of liquid crystals for heat imaging is known. These methods generally convert infrared or electrical energy into a heat pattern and thereafter apply the heat pattern to the liquid crystals to provide a color image corresponding to the heat pattern.
Liquid crystals have also been used for heat imaging of a microwave detector. For an example see the Augustine U.S. Pat. No. 3,693,084. A film of liquid crystals is in close proximity with a thin, continuous resistive layer. A microwave field incident on the composite structure generates alternating currents in the resistive layer which are in accord with the intensity of the microwave energy. The alternating electrical currents generate a heat pattern through dissipation in the resistive layer which corresponds to the intensity distribution of the incident microwave field. The heat pattern is imposed on the adjacent liquid crystals through thermal transfer. Since the liquid crystals assume particular colors in accordance with the temperature of the crystals, they will respond chromatically to incident microwave energy levels. Thus, a visible color pattern is produced representative of the pattern of intensity distribution of the microwave field. The resistive layer of the detector is constructed such that it has a resistivity of 377 ohms per square so that the impedance of the composite structure equals that of free space. Thus, microwave energy having a direction of propagation perpendicular to the resistive layer is theoretically absorbed therein. As a result of the absorption, the temperature of the detector is a direct quantitative measure of the power level of the incident microwave energy and may be determined by calibration of the detector. Calibration is accomplished by passing a direct current through the resistive layer. Microwave power and DC calibration current power of equal amounts provide the same film temperature. A reference detector may be used along with the main detector for mixed fields of infrared and microwave energy, arranged so that both absorb infrared energy while only the main detector also absorbs microwave energy. Calibration current through the reference detector to equalize the liquid crystal displays provides a measure of only the microwave energy.